Portable electronic devices such as cellular phones and laptop computers are in widespread use. A key feature of these devices is that they are battery operated, which allows them to be used in locations where AC power is not available. It is also important that the batteries be capable of powering the device for long periods of time and of being recharged in minimal periods, to maximize the time during which the device is available for use.
A particular problem occurs when the device is operated while its battery is being recharged. The AC/DC converter, commonly called an "AC adapter", that is used to supply DC power to the device is capable of delivering a certain amount of power. The battery charger, typically a DC/DC stepdown converter, is designed to deliver a fixed constant current to the battery, the magnitude of the current being determined by such factors as the battery characteristics, the AC adapter power level, and the charge time requirement. Considerations of size, weight and cost prevent the AC adapter from being made large enough to supply all of the power necessary to both operate the device and charge the battery simultaneously. Thus, some technique must be employed to assure that the device has sufficient power to operate in this situation. Ideally, the device would receive whatever power is necessary to operate it and all of the remaining power that the AC adapter is capable of delivering would be used to recharge the battery.
Several techniques have been used to address this problem. The simplest is illustrated in FIG. 1. Shown in FIG. 1 are an AC adapter 10, which could be any of a variety of AC/DC converters that are available on the market. The input terminal of AC adapter 10 is connected to a power main. The output terminal of AC adapter 10 is connected to a load 11, which could be, for example, a laptop computer, and through a battery charger 12 (a DC/DC stepdown converter) to a rechargeable battery 13, which powers load 11 when AC adapter 10 is not being used.
Since the voltage of rechargeable battery 13 varies from, for example, 2.7 volts when it is fully discharged to 4.2 volts when it is fully charged, battery charger 12 in the form of a DC/DC stepdown converter is necessary to ensure that the voltage supplied to the input terminal of battery 13 is at the proper level. The output voltage of AC adapter 10 is fixed at, for example, 5 volts .+-.5%. The structure of battery charger 12 is well known to those skilled in the art and will not be described in detail here. See, e.g., P. T. Krein, Elements of Power Electronics, Oxford University Press (1998). A driver 120 supplies signals over the lines designated DH and DL to the gate terminals, respectively of a high-side MOSFET 121 and a low-side MOSFET 122, which are connected in series between the input terminal of battery charger 12 and ground. The signals from driver 120 turn on MOSFETs 121 and 122 on and off in sequence, MOSFET 121 being on when MOSFET 122 is off, and vice-versa. To prevent current shoot-through from AC adapter 10 to ground, there is a "break-before-make" interval between the instant that one of MOSFETs 121 or 122 is turned off and the other MOSFET is turned on. The current delivered by battery charger 12 is directly related to the percentage of the time that MOSFET 121 is turned on (sometimes referred to as the duty cycle).
The voltage at the common node 123 between MOSFETs 121 and 122 therefore alternates between the output voltage of AC adapter 10 and ground. This voltage is fed to a series LC circuit containing inductor 124 and capacitor 125, which alternately store and deliver energy such that a generally constant current is generated on line 126 to battery 13.
Battery charger 12 also contains circuitry which allows the size of the current delivered to battery 13 to be set. A current detector 127 connected to line 126 generates a signal indicating the size of the current in line 126. This signal is delivered over a feedback loop 128 to a charge current control 129. Charge current control 129 has a charge current set input which is set to the desired current to be delivered to battery 13. Charge current set 126 compares the set current against the current detected by detector 127 and delivers an error signal which causes driver to adjust the duty cycle of the pulses delivered to MOSFETs 121 and 122 in such a way that the current on line 126 equals the set current.
In the circuitry shown in FIG. 1 the condition of the load 11 being operative is detected and the driver 120 and hence battery charger 12 is simply disabled in this situation. This solves the problem, but the battery is not charged at all whenever the load is on, thereby forfeiting valuable charge time that might be available.
An alternative solution is illustrated in FIG. 2. Here, instead of disabling the driver 120, the current setting in charge current control 129 is adjusted such that battery charger 12 delivers a reduced fixed current to battery 13 when the load 11 is operating. While this allows some charging to take place while the load is operative, the reduced fixed current delivered to the battery must be set based on the assumption that the load is drawing full power. Thus, when the load is not drawing full power, the power delivered to the battery is lower than AC adapter 10 is capable of delivering.
A more sophisticated solution is illustrated in FIG. 3. A current sense resistor 30 is connected to the output terminal of AC adapter 10. A current sense unit 32 reads the voltage drop across current sense resistor 30 and sends a signal representing the magnitude of the current to a compare unit 33, where the signal is compared against a signal representing the rated current of AC adapter 10. If the current through resistor 30 is greater than the rated current, less a safety margin, charge current control 129 reduces the current delivered by battery charger 12 to battery 13. With this arrangement a considerable safety margin must be applied and thus not all the available power is actually used. Moreover, the circuitry must be set for the rated output current of each AC adapter with which it is used; and sensing the size of a current, particularly at the high output voltages of many AC adapters, tends to be expensive. U.S. Pat. No. 5,939,862 describes an arrangement somewhat similar to the one shown in FIG. 3.